1. Field of the Invention
The invention pertains to comparators whose threshold potentials are independent of process and temperature variations. In preferred embodiments, the invention is a comparator whose hysteresis characteristics and threshold potentials are independent of process, temperature, and supply voltage variations, and depend only on a reference potential and on ratios of resistances of pairs of resistors.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a conventional comparator circuit, which is typically implemented as an integrated circuit (or portion of an integrated circuit). In FIG. 1, voltage source 6 maintains the inverting input of comparator 8 at reference voltage V.sub.ref above ground. Resistor R3 is connected between the output and noninverting input of comparator 8. Resistor R1 is connected between the circuit's input node (which is maintained at input potential V.sub.in) and comparator 8's noninverting input, and resistor R2 is connected between comparator 8's noninverting input and ground.
Comparator 8 has two states: a first state (in which its output potential V.sub.o has a first value V.sub.o =V.sub.oL), which it enters when the potential at it's noninverting input falls below a first threshold potential; and a second state (in which its output potential V.sub.o has a second value V.sub.o =V.sub.oH), which it enters when the potential at it's noninverting input rises to a second threshold potential. The FIG. 1 circuit is subject to hysteresis in the sense that the first threshold potential differs from the second threshold potential. Specifically, the second threshold potential is: EQU V.sub.LH =V.sub.in =V.sub.ref (1+R1/R2+R1/R3)-V.sub.oL (R1/R3);
and the first threshold potential is: EQU V.sub.HL =V.sub.in =V.sub.ref (1+R1/R2+R1/R3)-V.sub.oH (R1/R3).
Since the threshold potentials V.sub.LH and V.sub.HL depend on comparator 8's output potential (which is either V.sub.oH or V.sub.oL), they typically depend on process variations (which occur during manufacture of the FIG. 1 circuit), temperature variations (which occur during operation of the FIG. 1 circuit), and variations in the supply potential (potential V.sub.s provided to comparator 8). The hysteresis characteristics of the FIG. 1 circuit similarly depend on comparator 8's output potential (V.sub.oH or V.sub.oL), and thus also typically depends on process, temperature, and supply potential variations. It is impractical (and very difficult) to design an implementation of the FIG. 1 circuit in which neither the threshold potentials nor the hysteresis characteristics depend on process, temperature, or supply potential variations.